1. Technical Field of the Invention
This invention relates generally to the field of hydroprocessing catalysts for treatment of heavy cat naphtha (HCN) to produce desirable low sulfur hydrocarbon products without causing saturation of olefinic products or the formation of hydrogen sulfide. Specifically, the invention relates to a process for the removal of sulfur from a partially desulfurized naphtha stream.
2. Description of the Prior Art
In the petroleum industry, it is common for light gas oils, particularly middle distillate petroleum fuels, to contain sulfur species. Increasing concerns regarding pollutants present in the atmosphere have led to a desire to decrease the sulfur content of fuels used in engines, as engines and vehicles utilizing fuels which contain sulfur can produce emissions of nitrogen oxide, sulfur oxide and particulate matter. Government regulations have become more stringent in recent years with respect to allowable levels of the potentially harmful emissions.
Gasoline fuel can generally be prepared by blending several petroleum fractions. Typical refineries blend catalytically cracked gasoline (CCG), coker gasoline, straight run naphtha, reformate, isomerate and alkylate to produce gasoline fuel having selected specifications. In blended gasoline, CCG produced from a fluidized catalytic cracker or coker is responsible for a substantial portion of the sulfur content in the resulting blend. Removal of sulfur contained in the CCG is an important step in meeting the regulations on sulfur content in gasoline fuel.
In the field of petroleum refining, CCG is a stock of high-octane number gasoline containing a certain amount of olefin components. CCG is a gasoline fraction that can be obtained by catalytically cracking a heavy petroleum fraction as a stock oil, such as vacuum gas oil, and recovering and distilling the catalytically cracked products. In addition, CCG is a primary blending stock of automotive gasoline.
While some stock oils have small sulfur content and may be subjected to catalytic cracking without treatment, stock oil generally has a relatively high content of sulfur compounds. When untreated stock oil having a high sulfur content is subjected to catalytic cracking, the resulting CCG will also have high sulfur content.
One prior art technique for the removal of sulfur compounds from petroleum fractions is by catalytic hydrodesulfurization, also known as HDS, a process in which a sulfur containing petroleum fraction is contacted with a solid catalyst in the presence of hydrogen gas at elevated temperature and pressure to effectuate the removal of the sulfur from the petroleum fraction. Exemplary hydrodesulfurization catalysts can include an alumina support, molybdenum sulfide, cobalt sulfide and/or nickel sulfide. Catalytic activity of the hydrodesulfurization catalyst can be increased with the addition of a third or fourth element, such as for example, boron or phosphorous. However, removal of sulfur under relatively severe conditions requires a highly active and highly selective catalyst for use at high reaction temperatures and pressures.
Catalytic desulfurization generally takes place at elevated temperature and pressure in the presence of hydrogen, and may often result in the hydrogenation of other compounds, such as for example, olefin compounds, which may be present in the petroleum fraction which is being desulfurized. Hydrogenation of olefin products is generally undesirable as the olefins are partially responsible for providing higher octane ratings of the feedstock. Thus, hydrogenation of olefin compounds may result in a decreased overall octane rating for the feedstock. If there is significant loss of octane rating during the hydrodesulfurization of the hydrocarbon stream, because of saturation of olefin compounds, the octane loss must be compensated for by blending substantial amounts of reformate, isomerate and alkylate into the gasoline fuel. The blending of additional compounds to increase the octane rating is expensive and detrimental to the overall economy of the refining process.
Additionally, catalytic hydrodesulfurization can result in the formation of hydrogen sulfide as a byproduct. Hydrogen sulfide produced in this manner can recombine with species present in the hydrocarbon feed, and create additional or other sulfur containing species. Olefins are one exemplary species prone to recombination with hydrogen sulfide to generate organic sulfides and thiols. This reformation to produce organic sulfides and thiols can limit the total attainable sulfur content which may be achieved by conventional catalytic desulfurization.
Because HCN has a higher final boiling point than LCN and contains a larger amount of sulfur containing compounds (in particular benzothiophene), more severe hydrotreating conditions are typically required to attain a low sulfur content in the final product. The severe hydrotreating conditions can result in significant saturation of olefin compounds, even though the number of olefin compounds present in the HCN is relatively low as compared with the LCN. This results in a loss of octane number (RON).
Some conventional sulfur removal processes attempt to overcome the problem of octane number reduction by making use of the non-uniform distribution of olefins and sulfur-containing species across the naphtha boiling range. Typically in naphtha, olefins are most concentrated and the sulfur concentration is lowest in the fraction which boils between about 30° C. and 100° C., i.e., the light cat naphtha fraction. Sulfur species are most concentrated and the olefin concentration is relatively low in the heavy cat naphtha boiling range, typically between about 90° C. to about 230° C. Generally, in the HCN fraction, a large amount of sulfur species exist at higher distillation temperatures. Specifically, a high number of sulfur containing species exist in the portion of the HCN fraction boiling between approximately 150° C. and approximately 230° C. Sulfur species in the LCN fraction may be removed by caustic extraction without undesirable olefin saturation, while the HCN fractions generally require hydrotreating to remove the sulfur.
Because of the relatively high content of sulfur species in the higher boiling fraction of HCN, the industry currently only considers the HCN fraction between about 60° C. and about 160° C., excluding the portion of the HCN fraction having a boiling point between about 160° C. and about 230° C. because of the high sulfur content.
Therefore, improved products and methods for the removal of sulfur compounds from heavy cat naphtha fractions are needed which minimize both the saturation of olefins and the formation of hydrogen sulfide byproducts.